1. Field of the Invention
The present invention relates generally to automatic optical focusing methods and more particularly to a method and apparatus for automatically focusing high magnification microscopes on selected areas of interest in a field of view.
2. Description of the Prior Art
In a number of semiconductor or similar applications very high magnification microscopes are used to achieve the required resolution. Because of the theoretical diffraction limit, the objective lenses of these microscopes must have very high NAs (numerical apertures), i.e. Nas close to 1. At these very high Nas, automatic focusing of the microscope is particularly challenging since the depth of focus is very narrow. The problem becomes particularly difficult when the specimen under examination has considerable topology, and when coherence techniques are used and the depth of focus is even smaller than for conventional microscopes.
In typical semiconductor applications, including the examination of magnetic heads, only certain parts of the field of view are of interest where a certain critical dimension must be measured at specified points of each die. These repetitive measurements are accomplished by setting up a measurement process on particular locations on a sample chip of a wafer and then by automatically replicating the process for the entire batch of wafers, each containing many chips and possibly many measurements on each chip. The automatic process includes driving the stage to the correct measurement location, focusing at the level where a measurement must be taken and making the measurement and storing the reading in the computer.
The autofocus system of the KLA 5000 Coherence Probe, made by the assignee of the present invention, uses a single photodiode covering 1/3 of the linear field of view to detect the coherence of light reflected from an area of a surface to be inspected. Scanning the image in the Z direction, i.e. along the optical axis, provides interference intensity information that is measured by the photo-diode and later on analyzed by software to determine the best focus. However, the capability of this method is limited to a relatively flat area of interest. In applications where the area of interest is not flat, phase cancellation will occur, resulting in no information on the photo-diode and, eventually, inability to find the focal plane of the area of interest.
Another method known in the prior art uses a bright-field-focusing apparatus. In this method, the contrast of the image is maximized. This system is limited by the depth of focus of the lens. The larger the NA (numerical aperture), the shorter the depth of focus. Also, another problem with this method is that in some instances the contrast of the image may be too small to achieve focus.
Yet another bright field focusing method is called triangulation. In this method a very narrow beam (usually produced by a laser) is projected at an angle on the object to be focused. The location of the reflected beam is detected by a photocell array. The reflected beam returns to a different location on the array, depending on the distance of the object from the light source. The disadvantage of this method is that the beam does not pass "through the lens" (TTL) and has limited resolution. Such a non-TTL method poses some offset problems as well as other mechanical adjustment problems. Also, the resolution is limited by the number of elements on the linear array.
U.S. Pat. No. 4,340,306 issued to Balasubramanian discloses an optical system for surface topography measurement. The disclosed system characterizes an unknown test surface with respect to a known reference surface by using a dual beam interferometer having one wavefront reflected from the unknown test surface of a test object, while the other beam has reference wavefronts reflected from the known reference surface of a reference object.
Point coincidences between a return point on the reference surface and a test point on the test surface are indicated by the two reflected beams having a zero path difference. An array of points on the test surface are measured by scanning the interference pattern, point by point, and recording contrast variations by means of a multi-apertured CCD detector, with a CCD aperture corresponding to each test point. If a maximum contrast level is observed by an aperture, then the test point corresponding to that aperture is recorded as having a zero path difference with respect to the reference point on the reference surface. That is, there is coincidence between the test and reference points.
The Balasubramanian device is used to determine the surface profile of an object. Consequently, it must utilize the output from every detector in the detector array in order to generate a high resolution comparison between the test and the reference surfaces. The Balasubramanian device considers every portion of a surface as it performs its measurements.
In some applications, such as overlay measurements, one may, in succession, focus on the same location but on two different layers of the wafer. Methods and apparatus known in the prior art, for automated high-volume operation, perform this refocusing in an extremely time-consuming manner.
Finally, high resolution devices of the prior art, especially those that use interferometry, are extremely vulnerable to vibration.